Powder sintering device for moderator

ABSTRACT

A beam shaping assembly for neutron capture therapy includes a beam inlet, a target having nuclear reaction with an incident proton beam from the beam inlet to produce neutrons forming a neutron beam, a moderator adjoining to the target, a reflector surrounding the moderator, a thermal neutron absorber adjoining to the moderator, a radiation shield arranged inside the beam shaping assembly and a beam outlet. The material of the moderator is subjected to a powder sintering process using a powder sintering device so as to change powders or a power compact into blocks. The reflector leads the neutrons deviated from the main axis back. The thermal neutron absorber is used for absorbing thermal neutrons so as to avoid overdosing in superficial normal tissue during therapy. The radiation shield is used for shielding leaking neutrons and photons so as to reduce dose of the normal tissue not exposed to irradiation.

RELATED APPLICATION INFORMATION

This application is a continuation application of U.S. patentapplication Ser. No. 16/401,328, filed in the US on May 2, 2019, whichis a continuation application of U.S. patent application Ser. No.15/704,495, filed in the US on Sep. 14, 2017, now U.S. Pat. No.10,328,286, issued on Jun. 25, 2019, which is a continuation ofInternational Application No. PCT/CN2016/079568, filed on Apr. 18, 2016,which claims priority to Chinese Patent Application No. 201510222234.5,filed on May 4, 2015; Chinese Patent Application No. 201520281118.6,filed on May 4, 2015; Chinese Patent Application No. 201510579928.4,filed on Sep. 11, 2015; and Chinese Patent Application No.201520706407.6, filed on Sep. 11, 2015, the disclosures of which arehereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to a powder sintering device,and, more particularly, to a powder sintering device for a moderator.

BACKGROUND OF THE DISCLOSURE

As atomics moves ahead, such radiotherapy as Cobalt-60, linearaccelerators and electron beams has been one of major means to cancertherapy. However, conventional photon or electron therapy has beenundergone physical restrictions of radioactive rays; for example, manynormal tissues on a beam path will be damaged as tumor cells aredestroyed. On the other hand, sensitivity of tumor cells to theradioactive rays differs greatly, so in most cases, conventionalradiotherapy falls short of treatment effectiveness on radioresistantmalignant tumors (such as glioblastoma multiforme and melanoma).

For the purpose of reducing radiation damage to the normal tissuesurrounding a tumor site, target therapy in chemotherapy has beenemployed in the radiotherapy. While for high-radioresistant tumor cells,radiation sources with high RBE (relative biological effectiveness)including such as proton, heavy particle and neutron capture therapyhave also developed. Among them, the neutron capture therapy combinesthe target therapy with the RBE, such as the boron neutron capturetherapy (BNCT). By virtue of specific grouping of boronatedpharmaceuticals in the tumor cells and precise neutron beam regulation,BNCT is provided as a better cancer therapy choice than conventionalradiotherapy.

BNCT takes advantage that the boron (¹⁰B)-containing pharmaceuticalshave high neutron capture cross section and produces ⁴He and ⁷Li heavycharged particles through ¹⁰B(n,α)⁷Li neutron capture and nuclearfission reaction. As illustrated in FIGS. 1 and 2, a schematic drawingof BNCT and a nuclear reaction formula of ¹⁰B (n,α) ⁷Li neutron captureare shown, the two charged particles, with average energy at about 2.33MeV, are of linear energy transfer (LET) and short-rangecharacteristics. LET and range of the alpha particle are 150keV/micrometer and 8 micrometers respectively while those of the heavycharged particle ⁷Li are 175 keV/micrometer and 5 micrometersrespectively, and the total range of the two particles approximatelyamounts to a cell size. Therefore, radiation damage to living organismsmay be restricted at the cells' level. When the boronatedpharmaceuticals are gathered in the tumor cells selectively, only thetumor cells will be destroyed locally with a proper neutron source onthe premise of having no major normal tissue damage.

BNCT is also well known for binary cancer therapy, for its effectivenessdepending on the concentration of the boronated pharmaceuticals and thenumber of the thermal neutrons at the tumor site. Thus, besidesdevelopment of the boronated pharmaceuticals, improvement of flux andquality of the neutron source plays a significant role in BNCTresearches.

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

SUMMARY

In order to improve flux and quality of the neutron source, an aspect ofthe present disclosure provides a beam shaping assembly for neutroncapture therapy includes: a beam inlet; a target, wherein the target hasnuclear reaction with an incident proton beam from the beam inlet toproduce neutrons, and wherein the neutrons form a neutron beam defininga main axis; a moderator adjoining to the target, wherein the neutronsare moderated by the moderator to epithermal neutron energies, thematerial of the moderator is prepared by mixing a mixture containing oneor more of PbF₄, Al₂O₃, AlF₃, CaF₂ and MgF₂ and a ⁶Li element-containingmaterial accounting for 0.1 to 5% in percentage by weight of themixture, and wherein the material of the moderator is subjected to apowder sintering process using a powder sintering device so as to changepowders or powder compacts into blocks; a reflector surrounding themoderator, wherein the reflector leads the neutrons deviated from themain axis back to enhance epithermal neutron beam intensity; a thermalneutron absorber adjoining to the moderator, wherein the thermal neutronabsorber is used for absorbing thermal neutrons so as to avoidoverdosing in superficial normal tissue during therapy; a radiationshield arranged inside the beam shaping assembly, and wherein theradiation shield is used for shielding leaking neutrons and photons soas to reduce dose of the normal tissue not exposed to irradiation; and abeam outlet.

Implementations of this aspect may include one or more of the followingfeatures.

The incident proton beam is accelerated by means of an accelerator toovercome coulomb repulsion energy of a target atomic nucleus andgenerate nuclear reaction with the target to produce neutrons, and thetarget is made of a metal material.

An epithermal neutron energy range ranges from 0.5 eV to 40 keV, athermal neutron energy range is below 0.5 eV, and a fast neutron energyrange is above 40 keV, and the beam shaping assembly reduces thequantity of thermal neutrons and fast neutrons; and wherein thereflector is made of a material having a high neutron reflectionability, and the thermal neutron absorber is made of a material having across section for acting with thermal neutrons.

More particularly, the reflector is made of at least one of Pb or Ni.

More particularly, the thermal neutron absorber is made of ⁶Li, and anair passage is arranged between the thermal neutron absorber and thebeam outlet.

More particularly, the radiation shield may include a photon shield anda neutron shield.

Further, an outer surface of the moderator may include a first taperedsection and a second tapered section adjoining to the first taperedsection, and a tapering direction of the first tapered section isopposite to a tapering direction of the second tapered section, andwherein the first tapered section may include a first side and a secondside facing away from the beam outlet and is tapered gradually from thesecond side towards the first side, and the second tapered section mayinclude a third side and a fourth side facing the beam outlet and istapered gradually from the third side towards the fourth side.

More particularly, the first side defines a first diameterperpendicularly to the main axis, the second side and the third sidedefines a second diameter perpendicularly to the main axis and thefourth side defines a third diameter perpendicularly to the main axis,the first diameter is 1 cm to 20 cm in length, the second diameter is 30cm to 100 cm in length, the third diameter is 1 cm to 50 cm in length,and wherein multiple sintered blocks are connected to form the moderatorand a density of the moderator is 80 to 100 percent of theoreticaldensity.

Further, the powder sintering device is a hot-press sintering device ora spark plasma sintering device, and the powder sintering process is ahot-press sintering process or a spark plasma sintering process.

More particularly, the hot-press sintering device may include a heatingfurnace, a pressing assembly arranged in the heating furnace, a mold,powders or powder compacts loaded in the mold, and a controller forcontrolling the normal operation of the hot-press sintering device, andthe hot-press sintering process may include the following steps: fillingthe mold with a predetermined amount of powders or powder compacts;starting the hot-press furnace to preset pressure and temperatureparameters; moving the pressing assembly to press the powders or powdercompacts in the mold; controlling the hot-press sintering device by thecontroller to be under the condition of normal operation; and switchingon power to sinter the powders or powder compacts into blocks.

More particularly, the spark plasma sintering device may include a firstelectrode, a second electrode, a conductive mold arranged between thefirst electrode and the second electrode, a pulse current generator forproviding pulse current for the mold, a pressing assembly with apressing member for pressing, and a controller for controlling the pulsecurrent generator and the pressing assembly, wherein at least one of thefirst electrode and the second electrode is moved with respect to theother one, and at least one of the first electrode and the secondelectrode is connected to the pressing assembly, so that the powders orpowder compacts in the mold are pressed; and the spark plasma sinteringprocess comprises the following steps: filling the mold with apredetermined amount of powders or powder compacts; moving at least oneof the first electrode and the second electrode to press the powders orthe powder compacts in the mold; switching on the pulse currentgenerator to conduct electricity to the conductive mold, so that plasmais generated, and thereby the surfaces of the powders or the powdercompacts are activated and heat; and sintering the powders or the powdercompacts into blocks.

Further, the spark plasma sintering device further includes a positionmeasurement system for measuring the position of the pressing member, anatmosphere control system for controlling atmosphere in the mold, awater cooling system for cooling the spark plasma sintering device, anda temperature measurement system for measuring temperature in the sparkplasma sintering device, and the spark plasma sintering process furtherincludes the following steps: controlling the position measurementsystem by the controller in order to ensure the normal operation of theposition measurement system, controlling the atmosphere control systemby the controller in order to ensure that atmosphere in the mold isunder the condition of normal operation, controlling the water coolingsystem by the controller in order to ensure the normal operation of thewater cooling system, and controlling the temperature measurement systemby the controller in order to ensure that temperature in the sparkplasma sintering device is under the condition of normal operation.

In another aspect of the present disclosure, a beam shaping assembly forneutron capture therapy is provided for improving flux and quality ofthe neutron source. The beam shaping assembly includes a beam inlet; atarget, wherein the target has nuclear reaction with an incident protonbeam from the beam inlet to produce neutrons, and wherein the neutronsform a neutron beam defining a main axis; a moderator adjoining to thetarget, wherein the neutrons are moderated by the moderator toepithermal neutron energies, the material of the moderator is preparedfrom a material containing at least one of LiF, Li₂CO₃, Al₂O₃, AlF₃,CaF₂ and MgF₂, wherein the material of the moderator is subjected to apowder sintering process using a powder sintering device so as to changepowders or powder compacts into blocks; a reflector surrounding themoderator, wherein the reflector leads the neutrons deviated from themain axis back to enhance epithermal neutron beam intensity; a thermalneutron absorber adjoining to the moderator, wherein the thermal neutronabsorber is used for absorbing thermal neutrons so as to avoidoverdosing in superficial normal tissue during therapy; a radiationshield arranged inside the beam shaping assembly, wherein the radiationshield is used for shielding leaking neutrons and photons so as toreduce dose of the normal tissue not exposed to irradiation; and a beamoutlet.

Further, the outer surface of the moderator may include the firsttapered section and a second tapered section adjoining to the firsttapered section, and a tapering direction of the first tapered sectionis opposite to a tapering direction of the second tapered section, andwherein the first tapered section may include a first side and a secondside facing away from the beam outlet and is tapered gradually from thesecond side towards the first side, and the second tapered section mayinclude a third side and a fourth side facing the beam outlet and istapered gradually from the third side towards the fourth side.

More particularly, the first side defines a first diameterperpendicularly to the main axis, the second side and the third sidedefines a second diameter perpendicularly to the main axis and thefourth side defines a third diameter perpendicularly to the main axis,the first diameter is 1 cm to 20 cm in length, the second diameter is 30cm to 100 cm in length, the third diameter is 1 cm to 50 cm in length,and wherein multiple sintered blocks are connected to form the moderatorand a density of the moderator is 80 to 100 percent of theoreticaldensity.

Further, the powder sintering device is a hot-press sintering device ora spark plasma sintering device, and the powder sintering process is ahot-press sintering process or a spark plasma sintering process. Whereinthe hot-press sintering device includes a heating furnace, a pressingassembly arranged in the heating furnace, a mold, powders or powdercompacts loaded in the mold, and a controller for controlling the normaloperation of the hot-press sintering device, and the hot-press sinteringprocess includes the following steps: filling the mold with apredetermined amount of powders or powder compacts; starting thehot-press furnace to preset pressure and temperature parameters; movingthe pressing assembly to press the powders or powder compacts in themold; controlling the hot-press sintering device by the controller to beunder the condition of normal operation; and switching on power tosinter the powders or powder compacts into blocks. And wherein the sparkplasma sintering device includes a first electrode, a second electrode,a conductive mold arranged between the first electrode and the secondelectrode, a pulse current generator for providing pulse current for themold, a pressing assembly with a pressing member for pressing, and acontroller for controlling the pulse current generator and the pressingassembly, wherein at least one of the first electrode and the secondelectrode is moved with respect to the other one and connected to thepressing assembly, so that the powders or powder compacts in the moldare pressed; and the spark plasma sintering process includes thefollowing steps: filling the mold with a predetermined amount of powdersor powder compacts; moving at least one of the first electrode and thesecond electrode to press the powders or the powder compacts in themold; switching on the pulse current generator to conduct electricity tothe conductive mold, so that plasma is generated, and thereby thesurfaces of the powders or the powder compacts are activated and heat;and sintering the powders or the powder compacts into blocks.

More particularly, the spark plasma sintering device further includes aposition measurement system for measuring the position of the pressingmember, an atmosphere control system for controlling atmosphere in themold, a water cooling system for cooling the spark plasma sinteringdevice, and a temperature measurement system for measuring temperaturein the spark plasma sintering device, and the spark plasma sinteringprocess further includes the following steps: controlling the positionmeasurement system by the controller in order to ensure the normaloperation of the position measurement system, controlling the atmospherecontrol system by the controller in order to ensure that atmosphere inthe mold is under the condition of normal operation, controlling thewater cooling system by the controller in order to ensure the normaloperation of the water cooling system, and controlling the temperaturemeasurement system by the controller in order to ensure that temperaturein the spark plasma sintering device is under the condition of normaloperation.

In yet another aspect of the present disclosure, a beam shaping assemblyfor neutron capture therapy is provided for improving flux and qualityof the neutron source. The beam shaping assembly for neutron capturetherapy includes a beam inlet; a target, wherein the target has nuclearreaction with an incident proton beam from the beam inlet to produceneutrons, and wherein the neutrons form a neutron beam defining a mainaxis; a moderator adjoining to the target, wherein the neutrons aremoderated by the moderator to epithermal neutron energies, the materialof the moderator is prepared from a material containing at least one ofLiF, Li₂CO₃, Al₂O₃, AlF₃, CaF₂ and MgF₂, wherein the material of themoderator is subjected to a powder sintering process using a powdersintering device so as to change powders or powder compacts into blocks,and wherein the powder sintering device is a hot-press sintering deviceor a spark plasma sintering device, and the powder sintering process isa hot-press sintering process or a spark plasma sintering process; areflector surrounding the moderator, wherein the reflector leads theneutrons deviated from the main axis back to enhance epithermal neutronbeam intensity; a thermal neutron absorber adjoining to the moderator,wherein the thermal neutron absorber is used for absorbing thermalneutrons so as to avoid overdosing in superficial normal tissue duringtherapy; a radiation shield arranged inside the beam shaping assembly,wherein the radiation shield is used for shielding leaking neutrons andphotons so as to reduce dose of the normal tissue not exposed toirradiation; and a beam outlet.

Further, the outer surface of the moderator may include the firsttapered section and a second tapered section adjoining to the firsttapered section, and a tapering direction of the first tapered sectionis opposite to a tapering direction of the second tapered section, andwherein the first tapered section may include a first side and a secondside facing away from the beam outlet and is tapered gradually from thesecond side towards the first side, and the second tapered section mayinclude a third side and a fourth side facing the beam outlet and istapered gradually from the third side towards the fourth side, andwherein the first side defines a first diameter perpendicularly to themain axis, the second side and the third side defines a second diameterperpendicularly to the main axis and the fourth side defines a thirddiameter perpendicularly to the main axis, the first diameter is 1 cm to20 cm in length, the second diameter is 30 cm to 100 cm in length, thethird diameter is 1 cm to 50 cm in length, and wherein multiple sinteredblocks are connected to form the moderator and a density of themoderator is 80 to 100 percent of theoretical density.

More particularly, the hot-press sintering device may include a heatingfurnace, a pressing assembly arranged in the heating furnace, a mold,powders or powder compacts loaded in the mold, and a controller forcontrolling the normal operation of the hot-press sintering device, andthe hot-press sintering process may include the following steps: fillingthe mold with a predetermined amount of powders or powder compacts;starting the hot-press furnace to preset pressure and temperatureparameters; moving the pressing assembly to press the powders or powdercompacts in the mold; controlling the hot-press sintering device by thecontroller to be under the condition of normal operation; and switchingon power to sinter the powders or powder compacts into blocks; andwherein the spark plasma sintering device may include a first electrode,a second electrode, a conductive mold arranged between the firstelectrode and the second electrode, a pulse current generator forproviding pulse current for the mold, a pressing assembly with apressing member for pressing, and a controller for controlling the pulsecurrent generator and the pressing assembly, wherein at least one of thefirst electrode and the second electrode is moved with respect to theother one and connected to the pressing assembly, so that the powders orpowder compacts in the mold are pressed; and the spark plasma sinteringprocess may include the following steps: filling the mold with apredetermined amount of powders or powder compacts; moving at least oneof the first electrode and the second electrode to press the powders orthe powder compacts in the mold; switching on the pulse currentgenerator to conduct electricity to the conductive mold, so that plasmais generated, and thereby the surfaces of the powders or the powdercompacts are activated and heat; and sintering the powders or the powdercompacts into blocks.

The term ‘cylindrical’ or ‘cylindrical section’ referred in theembodiment of the present disclosure is an element with the contour in asubstantially unchanged trend from one side to the other side along theillustrated direction. One of contour lines may be a line segment, likea corresponding one of the cylinder, or may be a high-curvature arcapproximate to the line segment, like a corresponding one of a spherewith high curvature. The integral surface of the contour may becontinuously connected or not if the surface of the cylinder or thehigh-curvature sphere is provided with many protrusions and grooves.

The term ‘tapered’ or ‘tapered section’ referred in the embodiment ofthe present disclosure is an element with the contour in a taperingtrend from one to the other side along the illustrated direction. One ofcontour lines may be a line segment, like a corresponding one of thecone, or may be an arc, like a corresponding one of the sphere, and theintegral surface of the contour may be continuously connected or not ifthe surface of the cone shape or the spherical shape is provided withplenty of protrusions and grooves.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of boron neutron capture reaction.

FIG. 2 is a nuclear reaction formula of ¹⁰B (n,α) ⁷Li neutron capture.

FIG. 3 is a schematic view of the beam shaping assembly for neutroncapture therapy in the first embodiment of the present disclosure,wherein a gap channel is arranged between the moderator and thereflector.

FIG. 4 is a schematic view of the beam shaping assembly for neutroncapture therapy in the second embodiment of the present disclosure,wherein the outer surface of the moderator includes the first taperedsection and a second tapered section adjoining to the first taperedsection, and a tapering direction of the first tapered section isopposite to a tapering direction of the second tapered section, and thegap channel in the first embodiment is filled with materials of themoderator.

FIG. 5 is a schematic view of the beam shaping assembly for neutroncapture therapy in the third embodiment of the present disclosure,wherein the outer surface of the moderator includes the first taperedsection and a second tapered section adjoining to the first taperedsection, and a tapering direction of the first tapered section isopposite to a tapering direction of the second tapered section, and thegap channel in the first embodiment is filled with materials of thereflector.

FIG. 6 is a double-differential graph of neutron yield from neutronenergy and neutron angle.

FIG. 7 is a schematic view of the beam shaping assembly for neutroncapture therapy in the fourth embodiment of the present disclosure,wherein the moderator is cylindrical.

FIG. 8 is a schematic view of the beam shaping assembly for neutroncapture therapy in the fifth embodiment of the present disclosure,wherein the outer surface of the moderator includes a cylindricalsection and the first tapered section adjoining to the cylindricalsection.

FIG. 9 is a schematic view of a preparation device for a moderatormaterial in one embodiment of the present disclosure, wherein thepreparation device is a spark plasma sintering device.

FIG. 10 is a schematic view of a preparation device for a moderatormaterial in one embodiment of the present disclosure, wherein thepreparation device is a hot-press sintering device.

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure. Correspondingreference numerals indicate corresponding parts throughout the severalviews of the drawings.

DETAILED DESCRIPTION

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Neutron capture therapy (NCT) has been increasingly practiced as aneffective cancer curing means in recent years, and BNCT is the mostcommon. Neutrons for NCT may be supplied by nuclear reactors oraccelerators. Take AB-BNCT for example, its principal componentscomprise, in general, an accelerator for accelerating charged particles(such as protons and deuterons), a target, a heat removal system and abeam shaping assembly. The accelerated charged particles interact withthe metal target to produce the neutrons, and suitable nuclear reactionsare always determined according to such characteristics as desiredneutron yield and energy, available accelerated charged particle energyand current and materialization of the metal target, among which themost discussed two are ⁷Li (p, n) ⁷Be and ⁹Be (p, n) ⁹B and both areendothermic reaction. Their energy thresholds are 1.881 MeV and 2.055MeV respectively. Epithermal neutrons at a keV energy level areconsidered ideal neutron sources for BNCT. Theoretically, bombardmentwith lithium target using protons with energy slightly higher than thethresholds may produce neutrons relatively low in energy, so theneutrons may be used clinically without many moderations. However, Li(lithium) and Be (beryllium) and protons of threshold energy exhibit nothigh action cross section. In order to produce sufficient neutronfluxes, high-energy protons are usually selected to trigger the nuclearreactions.

The target, considered perfect, is supposed to have the advantages ofhigh neutron yield, a produced neutron energy distribution near theepithermal neutron energy range (see details thereinafter), littlestrong-penetration radiation, safety, low cost, easy accessibility, hightemperature resistance etc. But in reality, no nuclear reactions maysatisfy all requests. The target in these embodiments of the presentdisclosure is made of lithium. However, well known by those skilled inthe art, the target materials may be made of other metals besides theabove-mentioned.

Requirements for the heat removal system differ as the selected nuclearreactions. ⁷Li (p, n) ⁷Be asks for more than ⁹Be (p, n) ⁹B does becauseof low melting point and poor thermal conductivity coefficient of themetal (lithium) target. In these embodiments of the present disclosureis ⁷Li (p, n) ⁷Be.

No matter BNCT neutron sources are from the nuclear reactor or thenuclear reactions between the accelerator charged particles and thetarget, only mixed radiation fields are produced, that is, beamscomprise neutrons and photons having energies from low to high. As forBNCT in the depth of tumors, except the epithermal neutrons, the morethe residual quantity of radiation ray is, the higher the proportion ofnonselective dose deposition in the normal tissue is. Therefore,radiation causing unnecessary dose should be lowered down as much aspossible. Besides air beam quality factors, dose is calculated using ahuman head tissue prosthesis in order to understand dose distribution ofthe neutrons in the human body. The prosthesis beam quality factors arelater used as design reference to the neutron beams, which is elaboratedhereinafter.

The International Atomic Energy Agency (IAEA) has given five suggestionson the air beam quality factors for the clinical BNCT neutron sources.The suggestions may be used for differentiating the neutron sources andas reference for selecting neutron production pathways and designing thebeam shaping assembly, and are shown as follows:

Epithermal neutron flux>1×10⁹ n/cm²s

Fast neutron contamination<2×10⁻¹³ Gy-cm²/n

Photon contamination<2×10⁻¹³ Gy-cm²/n

Thermal to epithermal neutron flux ratio<0.05

Epithermal neutron current to flux ratio>0.7

Note: the epithermal neutron energy range is between 0.5 eV and 40 keV,the thermal neutron energy range is lower than 0.5 eV, and the fastneutron energy range is higher than 40 keV.

1. Epithermal Neutron Flux

The epithermal neutron flux and the concentration of the boronatedpharmaceuticals at the tumor site codetermine clinical therapy time. Ifthe boronated pharmaceuticals at the tumor site are high enough inconcentration, the epithermal neutron flux may be reduced. On thecontrary, if the concentration of the boronated pharmaceuticals in thetumors is at a low level, it is required that the epithermal neutrons inthe high epithermal neutron flux should provide enough doses to thetumors. The given standard on the epithermal neutron flux from IAEA ismore than 10⁹ epithermal neutrons per square centimeter per second. Inthis flux of neutron beams, therapy time may be approximately controlledshorter than an hour with the boronated pharmaceuticals. Thus, exceptthat patients are well positioned and feel more comfortable in shortertherapy time, and limited residence time of the boronatedpharmaceuticals in the tumors may be effectively utilized.

2. Fast Neutron Contamination

Unnecessary dose on the normal tissue produced by fast neutrons areconsidered as contamination. The dose exhibit positive correlation toneutron energy, hence, the quantity of the fast neutrons in the neutronbeams should be reduced to the greatest extent. Dose of the fastneutrons per unit epithermal neutron flux is defined as the fast neutroncontamination, and according to IAEA, it is supposed to be less than2*10⁻¹³ Gy-cm²/n.

3. Photon Contamination (Gamma-Ray Contamination)

Gamma-ray long-range penetration radiation will selectively result indose deposit of all tissues in beam paths, so that lowering the quantityof gamma-ray is also the exclusive requirement in neutron beam design.Gamma-ray dose accompanied per unit epithermal neutron flux is definedas gamma-ray contamination which is suggested being less than 2*10⁻¹³Gy-cm²/n according to IAEA.

4. Thermal to Epithermal Neutron Flux Ratio

The thermal neutrons are so fast in rate of decay and poor inpenetration that they leave most of energy in skin tissue after enteringthe body. Except for skin tumors like melanocytoma, the thermal neutronsserve as neutron sources of BNCT, in other cases like brain tumors, thequantity of the thermal neutrons has to be lowered. The thermal toepithermal neutron flux ratio is recommended at lower than 0.05 inaccordance with IAEA.

5. Epithermal Neutron Current to Flux Ratio

The epithermal neutron current to flux ratio stands for beam direction,the higher the ratio is, the better the forward direction of the neutronbeams is, and the neutron beams in the better forward direction mayreduce dose surrounding the normal tissue resulted from neutronscattering. In addition, treatable depth as well as positioning postureis improved. The epithermal neutron current to flux ratio is better oflarger than 0.7 according to IAEA.

The prosthesis beam quality factors are deduced by virtue of the dosedistribution in the tissue obtained by the prosthesis according to adose-depth curve of the normal tissue and the tumors. The threeparameters as follows may be used for comparing different neutron beamtherapy effects.

1. Advantage Depth

Tumor dose is equal to the depth of the maximum dose of the normaltissue. Dose of the tumor cells at a position behind the depth is lessthan the maximum dose of the normal tissue, that is, boron neutroncapture loses its advantages. The advantage depth indicatespenetrability of neutron beams. Calculated in cm, the larger theadvantage depth is, the larger the treatable tumor depth is.

2. Advantage Depth Dose Rate

The advantage depth dose rate is the tumor dose rate of the advantagedepth and also equal to the maximum dose rate of the normal tissue. Itmay have effects on length of the therapy time as the total dose on thenormal tissue is a factor capable of influencing the total dose given tothe tumors. The higher it is, the shorter the irradiation time forgiving a certain dose on the tumors is, calculated by cGy/mA-min.

3. Advantage Ratio

The average dose ratio received by the tumors and the normal tissue fromthe brain surface to the advantage depth is called as advantage ratio.The average ratio may be calculated using dose-depth curvilinearintegral. The higher the advantage ratio is, the better the therapyeffect of the neutron beams is.

To provide comparison reference to design of the beam shaping assembly,we also provide the following parameters for evaluating expressionadvantages and disadvantages of the neutron beams in the embodiments ofthe present disclosure except the air beam quality factors of IAEA andthe abovementioned parameters.

1. Irradiation time<=30 min (proton current for accelerator is 10 mA)

2. 30.0RBE-Gy treatable depth>=7 cm

3. The maximum tumor dose>=60.0RBE-Gy

4. The maximum dose of normal brain tissue<=12.5RBE-Gy

5. The maximum skin dose<=11.0RBE-Gy

Note: RBE stands for relative biological effectiveness. Since photonsand neutrons express different biological effectiveness, the dose aboveshould be multiplied with RBE of different tissues to obtain equivalentdose.

In order to improve flux and quality of neutron sources, the embodimentsof the present disclosure provides improvement of a beam shapingassembly for neutron capture therapy, preferably, improvement of a beamshaping assembly for AB-BNCT. As shown in FIG. 3, the beam shapingassembly 10 for neutron capture therapy in the first embodiment of thepresent disclosure comprises a beam inlet 11, a target 12, a moderator13 adjacent to the target 12, a reflector 14 surrounding the moderator13, a thermal neutron absorber 15 adjacent to the moderator 13, aradiation shield 16 and a beam outlet 17, wherein the radiation shield16 is set inside the beam shaping assembly 10. The target 12 has nuclearreaction with an incident proton beam from the beam inlet 11 to produceneutrons; the neutrons form a neutron beam, the neutron beam defines amain axis X, and the neutrons are moderated by the moderator 13 toepithermal neutron energies, and the reflector 14 leads the neutronsdeviated from the main axis X back to enhance epithermal neutron beamintensity; a gap channel 18 is placed between the moderator 13 and thereflector 14 so as to increase the epithermal neutron flux; the thermalneutron absorber 15 is used for absorbing thermal neutrons so as toavoid overdosing in superficial normal tissue during therapy; theradiation shield 16 is used for shielding the leaking neutrons andphotons so as to reduce dose of a normal tissue not exposed toirradiation.

AB-BNCT accelerates a proton beam using an accelerator. Preferably, thetarget 12 is made of a metal material, and the proton beam isaccelerated enough to overcome coulomb repulsion energy of a targetatomic nucleus and has ⁷Li (p, n) ⁷Be reaction with the target 12 toproduce neutrons. The beam shaping assembly 10 moderates the neutronsinto epithermal neutron energies and reduces the quantity of thermalneutrons and fast neutrons; the moderator 13 is made of a materialhaving a cross section for principally acting with fast neutrons buthardly acting with epithermal neutrons. Preferably, the moderator 13 ismade of at least one of D₂O, AlF₃, Fluental™, CaF₂, Li₂CO₃, MgF₂ andAl₂O₃. The reflector 14 is made of a material having high neutronreflection ability, and is made of at least one of Pb or Ni preferably.The thermal neutron absorber 15 is made of a material having a crosssection for acting with thermal neutrons and is made of ⁶Li preferably.An air passage 19 is placed between the thermal neutron absorber 15 andthe beam outlet 17. The radiation shield 16 comprises a photon shield161 and a neutron shield 162, and comprises a photon shield 161 made ofplumbum (Pb) and a neutron shield 162 made of polyethylene (PE)preferably.

An outer surface of the moderator 13 includes the first tapered sectionand a second tapered section adjoining to the first tapered section, anda tapering direction of the first tapered section is opposite to atapering direction of the second tapered section as shown in FIG. 3, theleft side of the out surface of the moderator 13 is shaped in a firsttapered section tapering gradually towards the left side, the right sideof the out surface of the moderator 13 is shaped in a second taperedsection tapering gradually towards the right side, and the two taperedsections connect to each other. Preferably, the left side of the outsurface of the moderator 13 is shaped in a cone tapering towards theleft side, and the right side may also be in other shapes adjacent tothe cone, such as cylinder. The reflector 14 tightly surrounds themoderator 13, and a gap channel 18 is placed between the moderator 13and the reflector 14. The so-called gap channel 18 is an empty areaunfilled by solid materials and allowing neutron beams to pass easily.The gap channel 18 may be an air or vacuum passage. The thermal neutronabsorber 15 arranged in the immediate vicinity of the moderator 13 ismade of a thin ⁶Li material layer, the photon shield 161 made of Pb inthe radiation shield 16 may be integrated with or separated from thereflector 14, the neutron shield 162 made of PE in the radiation shield16 may be arranged near the beam outlet 17. An air passage 19 is placedbetween the thermal neutron absorber 15 and the beam outlet 17, in thisarea, neutrons deviated from the main axis X may be kept leading back toenhance epithermal neutron beam intensity. A prosthesis B is arranged ata position about 1 cm away from the beam outlet 17. Well known by thoseskilled in the art, the photon shield 161 may be made of other materialsfor shielding photons; the neutron shield 162 also may be made of othermaterials or arranged in other places for shielding leaking neutrons.

For comparing difference between the beam shaping assemblies with andwithout the gap channel, referring to FIGS. 4 and 5, the gap channelfilled with the moderator in the second embodiment and the one filledwith the reflector in the third embodiment are shown. Referring to FIG.4 first, the beam shaping assembly 20 comprises a beam inlet 21, atarget 22, a moderator 23 adjoining to the target 22, a reflector 24surrounding the moderator 23, a thermal neutron absorber 25 adjacent tothe moderator 23, a radiation shield 26 and a beam outlet 27, whereinthe radiation shield 26 is set in the beam shaping assembly 20. Thetarget 22 has nuclear reaction with an incident photon beam from thebeam inlet 21 to produce neutrons, the neutrons form a neutron beam, theneutron beam defines a main axis X1, the neutrons are moderated by themoderator 23 to epithermal neutron energies, and the reflector 24 leadsthe neutrons deviated from the main axis X1 back to enhance theepithermal neutron beam intensity. An outer surface of the moderator 23includes the first tapered section and a second tapered sectionadjoining to the first tapered section, and a tapering direction of thefirst tapered section is opposite to a tapering direction of the secondtapered section as shown in FIG. 4, the left side of the out surface ofthe moderator 23 is shaped in a first tapered section tapering graduallytowards the left side, the right side of the out surface of themoderator 23 is shaped in a second tapered section tapering graduallytowards the right side, and the two tapered sections connect to eachother. The thermal neutron absorber 25 is used for absorbing thermalneutrons so as to avoid overdosing in superficial normal tissue duringtherapy; the radiation shield 26 is used for shielding leaking neutronsand photons so as to reduce dose of the normal tissue not exposed toirradiation.

Preferably, the target 22, the moderator 23, the reflector 24, thethermal neutron absorber 25 and the radiation shield 26 in the secondembodiment may be same as those in the first embodiment, wherein theradiation shield 26 comprises a photon shield 261 made of plumbum (Pb)and a neutron shield 262 made of polyethylene (PE), and the neutronshield 262 may be arranged at the beam outlet 27. An air passage 28 isplaced between the thermal neutron absorber 25 and the beam outlet 27. Aprosthesis B1 is arranged at a position about 1 cm away from the beamoutlet 27.

Referring to FIG. 5, the beam shaping assembly 30 comprises a beam inlet31, a target 32, a moderator 33 adjoining to the target 32, a reflector34 surrounding the moderator 33, a thermal neutron absorber 35 adjoiningto the moderator 33, a radiation shield 36 and a beam outlet 37, whereinthe radiation shield 36 is set in the beam shaping assembly 30. Thetarget 32 has nuclear reaction with an incident photon beam from thebeam inlet 31 to produce neutrons, the neutrons form a neutron beam, theneutron beam defines a main axis X2, the neutrons are moderated by themoderator 33 to epithermal neutron energies, and the reflector 34 leadsthe neutrons deviated from the main axis X2 back to enhance theepithermal neutron beam intensity. An outer surface of the moderator 33includes the first tapered section and a second tapered sectionadjoining to the first tapered section, and a tapering direction of thefirst tapered section is opposite to a tapering direction of the secondtapered section as shown in FIG. 5, the left side of the out surface ofthe moderator 33 is shaped in a first tapered section tapering graduallytowards the left side, the right side of the out surface of themoderator 33 is shaped in a second tapered section tapering graduallytowards the right side, and the two tapered sections connect to eachother. The thermal neutron absorber 35 is used for absorbing thermalneutrons so as to avoid overdosing in superficial normal tissue duringtherapy; the radiation shield 36 is used for shielding leaking neutronsand photons so as to reduce dose of the normal tissue not exposed toirradiation.

Preferably, the target 32, the moderator 33, the reflector 34, thethermal neutron absorber 35 and the radiation shield 36 in the thirdembodiment may be same as those in the first embodiment, wherein theradiation shield 36 comprises a photon shield 361 made of plumbum (Pb)and a neutron shield 362 made of polyethylene (PE), and the neutronshield 362 may be arranged at the beam outlet 37. An air passage 38 isplaced between the thermal neutron absorber 35 and the beam outlet 37. Aprosthesis B2 is arranged at a position about 1 cm away from the beamoutlet 37.

The followings are analog computation of the three embodiments by MCNPsoftware (a common-use software package developed by LosAlamos NationalLaboratory of the United States for computing neutrons, photons, chargedparticles or transporting coupled neutrons/photons/charged particles in3D complicated geometric structures).

Among them, Table 1 as follow shows performances of air beam qualityfactors in the three different embodiments (each item in the table iscalculated in the same unit above, so not repeat here and similarlyhereinafter):

TABLE 1 Air Beam Quality Factors Air beam quality Moderator-filledReflector-filled Gap factors gap channel gap channel channel Epithermalneutron 1.35E+09 1.38E+09 1.42E+09 flux Fast neutron 2.35E−13 2.58E−132.83E−13 contamination Photon contamination 1.22E−13 8.92E−14 8.02E−14Thermal to epithermal 0.03 0.02 0.02 neutron flux ratio Epithermalneutron 0.64 0.64 0.64 current to flux ratio

Table 2 shows dose performance in the three embodiments:

TABLE 2 Dose Performance Moderator-filled Reflector-filled Gap DosePerformance gap channel gap channel channel Advantage depth 10.9 10.911.0 Advantage depth dose 4.47 4.60 4.78 rate Advantage rate 5.66 5.695.68

Table 3 shows analog numerals of parameters for evaluating neutron beamdose performance in the three embodiments:

TABLE 3 Parameters for Evaluating Neutron Beam Dose PerformanceModerator-filled Reflector-filled Gap Parameters gap channel gap channelchannel Irradiation time 25.3 24.8 23.9 30.0RBE-Gy 7.7 7.7 7.7 treatabledepth Maximum 68.5 69.1 68.8 tumor dose Maximum dose 11.3 11.4 11.4 ofnormal brain tissue Maximum skin 11.0 11.0 11.0 dose

Note: it is observed from the three tables that the beam shapingassembly with the gap channel between the moderator and the reflectormay supply neutron beams having best therapeutic effect.

Neutrons produced from the lithium target feature higher forward averageenergy. As shown in FIG. 6, the average neutron energy is about 478 keVat a neutron scattering angle between 0° and 30° of and is only about290 keV between 30° and 180°. If forwardly travelling neutrons collidemuch with the moderator by changing the geometric shape of the beamshaping assembly, lateral neutrons may easily get to the beam outlet vialess collision, so theoretically, neutron moderation may be bestoptimized and the epithermal neutron flux may be improved effectively.Now from geometric shapes of the beam shaping assembly we may evaluateinfluences on the epithermal neutron flux from different geometricshapes of the beam shaping assembly.

FIG. 7 is a view of a geometric shape of the beam shaping assembly inthe fourth embodiment. The beam shaping assembly 40 comprises a beaminlet 41, a target 42, a moderator 43 adjoining to the target 42, areflector 44 surrounding the moderator 43, a thermal neutron absorber 45adjoining to the moderator 43, a radiation shield 46 and a beam outlet47, wherein the radiation shield 46 is set in the bean shaping assembly40. The target 42 has nuclear reaction with an incident photon beam fromthe beam inlet 41 to produce neutrons, the neutrons are moderated by themoderator 43 to epithermal neutron energies, and the reflector 44 leadsthe deviated neutrons back to enhance the epithermal neutron beamintensity. An out surface of the moderator 43 is columnar, preferably,cylindrical. The thermal neutron absorber 45 is used for absorbingthermal neutrons so as to avoid overdosing in superficial normal tissueduring therapy; the radiation shield 46 is used for shielding leakingneutrons and photons so as to reduce dose of the normal tissue notexposed to irradiation, and an air passage 48 is placed between thethermal neutron absorber 45 and the beam outlet 47.

FIG. 8 is a view of a geometric shape of the beam shaping assembly inthe fifth embodiment. The beam shaping assembly 50 comprises a beaminlet 51, a target 52, a moderator 53 adjoining to the target 52, areflector 54 surrounding the moderator 53, a thermal neutron absorber 55adjoining to the moderator 53, a radiation shield 56 and a beam outlet57, wherein the radiation shield 56 is set in the beam shaping assembly50. The target 52 has nuclear reaction with an incident photon beam fromthe beam inlet 51 to produce neutrons, the neutrons form a neutron beam,the neutron beam defines a main axis X3, the neutrons are moderated bythe moderator 53 to epithermal neutron energies, and the reflector 54leads the neutrons deviated from the main axis X3 back to enhance theepithermal neutron beam intensity. An out surface of the moderator 53includes a cylindrical section and a tapered section adjoining to thecylindrical section, the left side of the out surface of the moderator53 is shaped in a cylinder, the right side of the out surface of themoderator 53 is shaped in a cone tapering gradually from the right side,and the cylinder and the cone are adjacent to each other. The thermalneutron absorber 55 is used for absorbing thermal neutrons so as toavoid overdosing in superficial normal tissue during therapy; theradiation shield 56 is used for shielding leaking neutrons and photonsso as to reduce dose of the normal tissue not exposed to irradiation.

Preferably, the target 52, the moderator 53, the reflector 54, thethermal neutron absorber 55 and the radiation shield 56 in the fifthembodiment may be same as those in the first embodiment, wherein theradiation shield 56 comprises a photon shield 561 made of plumbum (Pb)and a neutron shield 562 made of polyethylene (PE), and the neutronshield 562 may be arranged at the beam outlet 57. An air passage 58 isplaced between the thermal neutron absorber 55 and the beam outlet 57. Aprosthesis B3 is arranged at a position about 1 cm away from the beamoutlet 57.

In the following, results of analog computation of the moderator with anout surface including two opposite tapered sections in the secondembodiment, the cylindrical moderator in the fourth embodiment and themoderator with an out surface including a cylindrical section and atapered section adjoining to the cylindrical section in the fifthembodiment by MCNP are shown.

Among them, Table 4 shows air beam quality factors in these threeembodiments:

TABLE 4 Air Beam Quality Factors A cylindrical Two section and oppositeAir beam quality Cylindrical a tapered tapered factors section sectionsections Epithermal neutron 7.14E+08 1.29E+09 1.35E+09 flux Fast neutron2.67E−13 2.40E−13 2.35E−13 contamination Photon contamination 1.72E−131.42E−13 1.22E−13 Thermal to epithermal 0.04 0.03 0.03 neutron fluxratio Epithermal neutron 0.69 0.64 0.64 current to flux ratio

Table 5 shows dose performance in these three embodiments:

TABLE 5 Dose Performance A cylindrical Two section and oppositeCylindrical a tapered tapered Dose Performance section section sectionsAdvantage depth 11.8 10.9 10.9 Advantage depth dose 2.95 4.28 4.47 rateAdvantage rate 5.52 5.66 5.66

Table 6 shows analog numerals of parameters for evaluating neutron beamdose performance in these three embodiments:

TABLE 6 Parameters for Evaluating Neutron Beam Dose Performance Acylindrical Two section and opposite Cylindrical a tapered taperedParameters section section sections Irradiation time (10 mA) 40.7 26.125.3 30.0RBE-Gy treatable 8.4 7.6 7.7 depth Maximum tumor dose 70.9 67.468.5 Maximum dose of 12.0 11.2 11.3 normal brain tissue Maximum skindose 11.0 11.0 11.0

Note: it is observed from these three tables that the out surface of themoderator may include at least one tapered section, and its neutronbeams may achieve better therapeutic effect.

The term ‘cylindrical’ or ‘cylindrical section’ referred in theembodiment of the present disclosure is an element with the contour in asubstantially unchanged trend from one side to the other side along theillustrated direction. One of contour lines may be a line segment, likea corresponding one of the cylinder, or may be a high-curvature arcapproximate to the line segment, like a corresponding one of a spherewith high curvature. The integral surface of the contour may becontinuously connected or not if the surface of the cylinder or thehigh-curvature sphere is provided with many protrusions and grooves.

The term ‘tapered’ or ‘tapered section’ referred in the embodiment ofthe present disclosure is an element with the contour in a taperingtrend from one to the other side along the illustrated direction. One ofcontour lines may be a line segment, like a corresponding one of thecone, or may be an arc, like a corresponding one of the sphere, and theintegral surface of the contour may be continuously connected or not ifthe surface of the cone shape or the spherical shape is provided withplenty of protrusions and grooves.

In an important aspect of source flux improvement, it is necessary todiscuss the preparation of the material of the moderator, and themoderator 13 is further elaborated hereinafter with the first embodimentand FIG. 3 as an example.

The moderator 13 shows a double-taper structure in which the directionsof the two tapered sections are completely opposite, the material of themoderator 13 is prepared from a material containing at least one ofAlF₃, CaF₂ and MgF₂, and the moderator 13 has a first diameter D1perpendicularly to the main axis X, a second diameter D2 perpendicularlyto the main axis X and a third diameter D3 perpendicularly to the mainaxis X. An opening is arranged at the first diameter D1 so as to containthe target 12, and the second diameter D2 is arranged as the maximumsize of the double-taper structure. For BNCT, in order to achieve anenough moderating effect, the first diameter D1 is 1 cm to 20 cm inlength, the second diameter D2 is 30 cm to 100 cm in length, and thethird diameter D3 is 1 cm to 50 cm in length; as a preference, the firstdiameter D1 is 10 cm in length, the second diameter D2 is 70 cm inlength, and the third diameter D3 is 30 cm in length. In order to obtainthe moderator 13 with such a large size, the density of its material is80 to 100 percent of theoretical density, and the preparation of thefollowing three types of moderator materials is provided.

1. Crystal Growing

MgF₂ is taken as an example first, and please further refer to inventionpatent application publication No. CN102925963A, which is completelyintroduced as a reference for crystal growing preparation herein. As acrystal growing method, usually a seed crystal and MgF₂-containingpowders are put into a crucible, and an MgF₂ monocrystal is grown in acertain way.

It should be especially noted that the so-called “monocrystal” heremeans a single crystal which is grown to form once, rather than a singlecrystalline grain (that is, there is only one crystalline form and onlyone crystalline grain is contained, and molecules and atoms in thecrystalline grain are all arranged regularly). It can be betterunderstood that such a single crystalline grain is different frommultiple crystalline grains (that is, the sizes and shapes of thecrystalline grains are different, moreover, orientations are in disorderas well, there are no distinct shapes, and the crystalline grains do notalso show anisotropy). The definition about “monocrystal” hereinafter isthe same as here.

Through research, PbF₄, AlF₃, CaF₂ and Al₂O₃ can also be prepared in asimilar way.

2. Powder Sintering

The powders or powder compacts of MgF₂, AlF₃ or CaF₂ is furthercombined, the powder grains will undergo physical and chemicalprocesses, such as mutual flowing, diffusion, dissolution andrecrystallization, in the process of sintering, consequently, thepowders is further compacted, and part or all of voids therein areeliminated. There can be a lot of sintering methods, such as solid-phasesintering, i.e., sintering temperature is lower than the melting pointof each component in the powder; liquid-phase sintering, i.e., if thereare two or more types of components in the powder compact, sintering maytake place above the melting point of a certain component, so a smallamount of liquid phase will appear in the powder compact duringsintering; hot-press sintering, i.e., during sintering, pressure isapplied to the powder so as to promote the process of powder compaction,and hot pressing is a technological process which combines the shapingand sintering of powder together to directly obtain a product; and sparkplasma sintering, i.e. a rapid sintering technique, i.e., by applyingON-OFF direct-current pulse voltage generated by a special powercontroller to a powdery test sample, a sintering promotion effect causedby a spark discharge phenomenon (the instantaneous generation ofhigh-temperature plasma) generated in powder at the initial stage ofpulse discharge can effectively utilized, besides the common sinteringpromotion effect (discharge impact pressure and Joule heating)engendered by discharge processing, to implement compaction through aninstantaneous high temperature field. The powder sintering device turnsthe material of the moderator into blocks from the powders or powdercompacts through a powder sintering process.

As known well by those skilled in the art, other sintering methods canalso implement the preparation of at least one or a mixture of more ofMgF₂, AlF₃ and CaF₂ as the material of the moderator. As a preference,hot-press sintering and spark plasma sintering are taken as embodimentsof powder sintering hereinafter.

MgF₂ powder or mixed powder in which ⁶LiF accounting for 0.1 to 5percent by weight of MgF₂ powder is added in MgF₂ is then taken as anexample for the introduction of powder sintering, and preferably, themixed powder in which ⁶LiF accounting for 0.1 to 5 percent of thepercentage by weight of the MgF₂ powder is added in MgF₂ is taken as anexample for the introduction of powder sintering.

The moderator plays a vital role in the beam shaping assembly,undertaking great responsibility for neutron moderation, and it needs toreduce fast neutron intensity as much as possible without excessivelymoderating neutrons into thermal neutrons, and, on the other hand, mustalso reduce γ rays derived in the process of moderation. A researchindicates that the intensity of γ rays can be effectively decreased byevenly adding a small amount of ⁶Li-containing material into themoderator, and although neutron intensity would be slightly decreased,original beam quality is still kept. In a further research, ⁶LiF powderaccounting for 0.1 to 5 percent by weight of MgF₂ powder is mixed intothe MgF₂ powder, and compared with the pure MgF₂ powder without the ⁶LiFpowder, the mixed powder can more effectively absorb thermal neutronsand effectively inhibit γ rays.

When the ⁶Li-containing material accounting for 0.1 to 5 percent byweight of the MgF₂ powder is mixed into the MgF₂ powder to form amoderator material, it is known well to those skilled in the art thatthe ⁶Li-containing material can be of any physical form which can beeasily mixed with the MgF₂ powder, for example, the ⁶Li-containingmaterial can be liquid or powder. The ⁶Li-containing material can be anycompound which can be easily mixed with the MgF₂ powder, and the⁶Li-containing material can be ⁶LiF or ⁶Li₂CO₃. As a preference, theMgF₂ powder and the ⁶LiF powders or powder compacts accounting for 0.1to 5 percent by weight of the MgF₂ powder are further combined, thepowder grains will undergo physical and chemical processes, such asmutual flowing, diffusion, dissolution and recrystallization, in theprocess of sintering, consequently, the powder is further compacted, andpart or all of voids therein are eliminated. There can be a lot ofsintering methods, such as solid-phase sintering, i.e., sinteringtemperature is lower than the melting point of each component in thepowder; liquid-phase sintering, i.e., if there are two or more types ofcomponents in the powder compact, sintering may take place above themelting point of a certain component, so a small amount of liquid phasewill appear in the powder compact during sintering; hot-press sintering,i.e., during sintering, pressure is applied to the powder so as topromote the process of powder compaction, and hot pressing is atechnological process which combines the shaping and sintering of powdertogether to directly obtain a product; and spark plasma sintering, i.e.a rapid sintering technique, i.e., by applying ON-OFF direct-currentpulse voltage generated by a special power controller on a powdery testsample, a sintering promotion effect caused by a spark dischargephenomenon (the instantaneous generation of high-temperature plasma)generated in powder at the initial stage of pulse discharge can beeffectively utilized, besides the common sintering promotion effect(discharge impact pressure and Joule heating) engendered by dischargeprocessing, to implement compaction through an instantaneous hightemperature field. The powder sintering device turns the material of themoderator into a block from the powders or powder compacts through apowder sintering process.

As known well by those skilled in the art, other sintering methods canalso implement the preparation of the material of the moderator byadding ⁶LiF powder into at least one or a mixture of MgF₂, AlF₃, CaF₂and PbF₄. As a preference, hot-press sintering and spark plasmasintering are taken as embodiments of powder sintering hereinafter.

2.1 Spark Plasma Sintering

Spark plasma sintering integrates plasma activation, hot pressing andresistance heating into a whole, temperature rise is rapid, sinteringtime is short, and sintering temperature is low, crystalline grains areuniform, the control of the microstructure of a sinter is benefited, thecompactness of the obtained material is high, and moreover, spark plasmasintering has the advantages of simplicity in operation, highrepeatability, high safety and reliability, space saving, energy saving,low cost, etc. Because spark plasma sintering applies strong pulsecurrent between powder grains, electrical field-induced anodes andcathodes exist between the powder grains, discharge takes place betweenthe grains under the effect of the pulse current, exciting plasma,high-energy particles generated by discharge impact the contactedportions of the grains, so that the substance produces an evaporationeffect, playing a purifying and activating role, electrical energy isstored in the dielectric layers of the clusters of grains, and thedielectric layers discharge electricity intermittently and rapidly.Since the pulse current exists in the powders or powder compacts andoccurs instantaneously and intermittently at high frequency, bothdischarge heat generated by the uncontacted portions of the powdergrains and Joule heat generated by the contacted portions of the powdergrains can greatly promote the diffusion of powder grain atoms, thediffusion coefficient is much greater than that under the normal hotpressing condition, and thereby the rapidness of powder sintering isachieved. Moreover, because of the addition of the pulse current, boththe discharging portions and Joule heating portions in the powder willmove rapidly, so that the sintering of the powders or powder compactscan be uniform. In the process of spark plasma sintering, as dischargeoccurs between the grains, local high temperature which is as high asthousands of degrees Celsius to ten thousand degrees Celsius isgenerated instantaneously, causing evaporation and melting of thesurfaces of the grains, as a result, necks are formed at the contactpoints of the grains, and because heat is immediately transferred fromthe heating center to the surfaces of the grains and diffused in alldirections, the necks are cooled rapidly, leading to vapor pressurelower than that of the other portions. The gas-phase substance isagglomerated on the necks to form evaporation-solidification transitionhigher than that in ordinary sintering methods, which is anotherimportant characteristic of the spark plasma sintering process. Becausepulse current heating and vertical unidirectional pressure act on thecrystalline grain, both bulk diffusion and grain boundary diffusion areenhanced, the process of sintering compaction is accelerated, andtherefore a high-quality sinter can be obtained with low temperature ina short time. The spark plasma sintering process can be regarded as aresult of the comprehensive effect of grain discharge, conductiveheating and pressing.

Refer to FIG. 9, it discloses a schematic view of a spark plasmasintering device. The spark plasma sintering device 100 includes a firstelectrode 101, a second electrode 102, a conductive mold 103 arrangedbetween the first electrode 101 and the second electrode 102, a pulsecurrent generator 104 for providing pulse current for the mold 103, apressing assembly 105 with pressing members 1051, 1052 for pressing thepowders or powder compacts, and a controller 106 for controlling thepulse current generator 104 and the pressing assembly 105, at least oneof the first electrode 101 and the second electrode 102 can be moved,and at least one of the pressing members 1051, 1052 can be moved; as apreference, the first electrode 101 and the pressing part 1051 arefixed, the second electrode 102 and the pressing part 1052 can be moved,and thereby powders or powder compacts 107 loaded in the mold 103 can bepressed. As a preference, the conductive mold 103 is of lead orgraphite. The spark plasma sintering device 100 further includes aposition measurement system 108 for measuring the position of thepressing assembly 105, an atmosphere control system 109 for controllingatmosphere in the mold 103, a water cooling system 111 for controlling awater-cooling vacuum chamber 110 to carry out cooling, and a temperaturemeasurement system 112 for measuring temperature in the spark plasmasintering device 100. Pulse current is applied to the mold 103 and thepowders or powder compacts 107; besides providing discharge impactpressure and Joule heat for sintering, a sintering promotion effectcaused by a spark discharge phenomenon (the instantaneous generation ofhigh-temperature plasma) generated in powder at the initial stage ofpulse discharge is further utilized to implement rapid sintering throughan instantaneous high temperature field, so that the powders or powdercompacts 107 are turned into blocks from the powder state, and theso-called blocks are integrally formed without needing, for example,putting together monocrystals by steps, such as grinding or polishing,to match the dimensions of the moderator, like the crystal growingmethod.

The spark plasma sintering device 100 utilizes direct-current pulsecurrent to be directly electrified for sintering and pressing, andcontrols the rate of temperature rise and sintering temperature byregulating the magnitude of the direct-current pulse current through thecontroller 106. The whole sintering process can be carried out under avacuum environment, or can be carried out in protective atmosphere, suchas oxygen or hydrogen.

Under oxygen atmosphere, because oxygen is adsorbed by the surface ofthe sinter or chemical reaction occurs, a cation vacancy typenon-stoichiometric compound is formed on the surface of the crystal,cation vacancies are increased, meanwhile, oxygen in closed aperturescan directly get into the crystal lattice, and is diffused along thesurface like oxygen ion vacancies, and diffusion and sintering areaccelerated. When sintering is controlled by cation diffusion, oxidizingatmosphere or oxygen partial pressure is high and favorable for theformation of the cation vacancies, promoting sintering; and whensintering is controlled by anion diffusion, reducing atmosphere or lowoxygen partial pressure will lead to the generation of oxygen ionvacancies and promote sintering.

When a sample is sintered under hydrogen atmosphere, as the radius ofthe hydrogen atom is very small, hydrogen is easy to diffuse andbeneficial to elimination of closed apertures, and when a type ofmaterial, such as alumina, is sintered under the hydrogen atmosphere, asinter sample which approximates theoretical density can be obtained.

Sintering temperature is one of key parameters in the process of plasmarapid sintering. The determination of a sintering temperature must takethe phase transformation of the sinter sample under high temperature,the growth rate of a crystalline grain, the requirement on the qualityof the sample and the requirement on the density of the sample intoconsideration. In general, as sintering temperature rises, the overallcompactness of a test sample tends to increase, this indicates thatsintering temperature has remarkable influence on the compactness degreeof the sample, and the higher sintering temperature is, the higher thespeed of substance transmission is in the process of sintering and theeasier the sample is to compact.

However, the higher temperature is, the higher the growth rate of acrystalline grain is and the poorer its mechanical properties are. Whentemperature is too low, the compactness of the sample is very low, andquality cannot meet a requirement. Due to the contradiction betweentemperature and crystalline grain size, an appropriate parameter isrequired in respect of temperature choice.

Normally, prolonging temperature keeping time under sinteringtemperature will promote the completion of sintering to differentdegrees and perfect the microstructure of the sample, which is moreobvious for sintering for a viscose flow mechanism while having lessinfluence on sintering for bulk diffusion and surface diffusionmechanism. In the process of sintering, normally, when temperature iskept for only one minute, the density of the sample reaches not lessthan 96.5 percent of theoretical density; as temperature keeping timeextends, the compactness of the sample increases, but the variationrange is not broad, and this indicates that although the temperaturekeeping time has certain influence on the compactness of the sample, theeffect of action is not remarkable. However, if the temperature keepingtime under the sintering temperature is prolonged unreasonably, thecrystalline grain will rapidly grow within the time, intensifyingsecondary recrystallization, which is adverse to a requirement on theproperties of the sample, and if the time is too short, it will cause adecrease in the compactness of the sample, so it is necessary to choosean appropriate temperature keeping time.

With an increase in the rate of temperature rise, the sample reaches arequired temperature within a short time, the growth time of thecrystalline grain will be greatly shortened, and this not only helps toinhibit the crystalline grain from growing up, so that a fine-grainedceramic with uniform size can be obtained, but also can save time andenergy and increase the utilization rate of the sintering device.However, due to the limitation of the device, an overhigh rate oftemperature rise will cause a destructive effect on the device. For thisreason, the rate of temperature rise should be increased as much aspossible within an allowable range. Nevertheless, it is reflected inmeasured experimental data that different from sintering temperature andtemperature keeping time, the influence of the rate of temperature riseon sample compactness shows an opposite result, that is, as the rate oftemperature rise increases, sample compactness shows a tendency ofcoarsening and gradually decreasing. Some scholars have suggested thatthis is because the increase of the rate of temperature rise nearsintering temperature is equivalent to the shortening of temperaturekeeping time, so sample compactness will decrease to a certain degree.In an actual high-temperature sintering process, the temperature riseprocess is normally divided into three stages, i.e. a stage from roomtemperature to about 600° C., a stage from 600° C. to about 900° C. anda stage from 900° C. to a sintering temperature: the first stage is apreparation stage, and the rate of temperature rise is relatively slow;the second stage is a controllable rapid temperature rise stage, and therate of temperature rise is normally controlled at 100 to 500(° C./min);the third stage is a buffering stage of temperature rise, temperature isslowly increased to the sintering temperature at this stage, thetemperature keeping time is normally one to seven minutes, a sinter iscooled along with the furnace after temperature keeping, and the coolingrate can reach 300° C./min.

After sufficient discharge treatment, powders are immediately pressed tobe shaped and sintered. The sintered material is severely plasticallydeformed under the combined action of resistance Joule heat andpressure, applying forming pressure can help to enhance the contactbetween the powder grains, enlarge sintering area, exhaust residual gasin the sintered powder and increase the strength, density and surfacesmoothness of a product. The magnitude of forming pressure is normallydetermined according to the compactness of the sintered powder andrequirements on the properties of the sintered material, such as densityand strength, and is normally within a range from 15 MPa to 30 MPa, andsometimes, may be as high as 50 MPa or even higher. Usually, the higherforming pressure is, the density of the sintered material is. Theduration of pressure application will also greatly affect the density ofthe sintered material, and depending on varieties of sintered materials,powder grain sizes and geometrical dimensions of the sintered materials,appropriate pressure application time may be different, and needs to bedetermined by experiments. An experiment proves that the duration ofpressure application is equal to or slightly greater than dischargetime, and this is a necessary condition to obtain a sintered materialwith the highest density. It is easy to understand from a sintering andsolid-phase reaction mechanism that the higher pressure is, the moretightly grains in a sample heap, mutual contact points and contact areasare enlarged, and sintering is accelerated. Thus, the sample can obtainbetter compactness, moreover, the crystalline grain can be effectivelyinhibited from growing up, and sintering temperature can be decreased.Therefore, chosen pressure is normally 30 Mpa to 50 MPa. Nevertheless, aresearch indicates that during sintering, when external pressure is 30MPa and 50 MPa, the difference between the compactnesses of the sampleis not great, and this suggests that the phenomenon that compactnessincreases along with pressure is only obvious within a certain range.

Compared with conventional sintering techniques, spark plasma sinteringhas the following advantages: sintering speed is high; materialmicrostructures are improved, and the properties of materials areincreased.

As known well by those skilled in the art, the mold can be produced byusing other conductive materials, the spark plasma sintering device canalso be so arranged that both electrodes are fixed, while only at leastone pressing member can move.

The main process flow of spark plasma sintering is divided into fourstages in total. First stage: initial pressure is applied to a powdersample to make the powder grains be in sufficient contact with oneanother, so that uniform and sufficient spark plasma can be generated inthe powder sample later; Second stage: pulse current is applied, thecontact points of the powder grains generate spark plasma under theeffect of the pulse current, and the grain surfaces generate a slightheat releasing phenomenon due to activation; Third stage: a pulse powersupply is switched off, resistance heating is carried out on the sampleuntil the sample reaches a predetermined sintering temperature and thecontraction of the sample is complete; Fourth stage: pressure isreleased. By reasonably controlling main technological parameters, suchas initial pressure, sintering time, forming time, pressure applicationduration, sintering temperature and the rate of temperature rise, amaterial with good comprehensive properties can be obtained.

Due to a bridging effect between the powder grains, they cannot be insufficient contact normally, so, in order to generate uniform andsufficient-discharge plasma in the sample during spark sintering andactivate the grain surfaces to the max to accelerate the sinteringcompaction process, appropriate initial pressure needs to be applied tothe sintered powders, so that the powder grains can be in sufficientcontact. The magnitude of initial pressure may be different, dependingon varieties of sintered powders and sizes and properties of sinters. Ifinitial pressure is too low, the discharge phenomenon will be onlylimited to part of the powders, leading to the partial melting of thepowders; if initial pressure is too high, discharge will be inhibited,and the sintering diffusion process will then be retarded. According toexisting literature, in order to make discharge proceed continuously andsufficiently, the initial pressure should not exceed 10 MPa normally.

When a powder test sample with good spark sintering conductivity isused, because resistance heating is carried out simultaneously from theoutside and inside of the sample, the sintering time is extremely shortor even instantaneous, but the length of the sintering time should bedifferent according to qualities, varieties and properties of powders,and is normally several seconds to several minutes; and when a large,difficult-to-melt metal powder material is sintered, the sintering timeis even up to tens of minutes. Sintering time has great influence on thedensity of the product, and in order to make the compaction processproceed sufficiently, a certain sintering time needs to be guaranteed.

It is generally believed that rapid temperature rise in the process ofspark plasma sintering is beneficial to the sintering of powders becauseit inhibits the non-compaction mechanism of the material and activatesthe compaction mechanism of the material, so increasing the rate oftemperature rise can make the compaction degree of the sample increased.

As a preference, the spark plasma sintering process includes thefollowing steps: filling the mold 103 with an appropriate amount ofpowders or powder compacts 107; moving the pressing assembly 105 topress the powders or powder compacts 107 in the mold 103; switching on,by utilizing the controller 106, the pulse current generator 104 toelectrify the mold 103, so that plasma is generated and the surfaces ofthe powder grains are activated and heat; and sintering the powders orpowder compacts 107 into blocks. The spark plasma sintering processfurther includes the following steps: the controller 106 controls theposition measurement system 108 to ensure the normal operation of theposition measurement system 108, the controller 106 controls theatmosphere control system 109 to ensure that atmosphere in the mold 103is under the condition of normal operation, the controller 106 controlsthe water cooling system 111 to ensure that it is under the condition ofnormal operation, and the controller 106 controls the temperaturemeasurement system 112 to ensure that temperature in the spark plasmasintering device 100 is under the condition of normal operation. Theso-called normal operation means that the spark plasma sintering devicedoes not generate visual, tactile or auditory warning signalsperceivable by the human being, such as the shining of a warningindicator light, the sounding of the warning indicator light, warningindicator vibration and the like.

2.2 Hot-Press Sintering

Hot-press sintering is a sintering method in which dry powder is loadedinto the mold, and is then pressed in a single-axis direction whilebeing heated, so that forming and sintering are complete at the sametime. The hot-press sintering technique is rich in production processes,and there are no unified specifications and standard for classificationat present. According to current situation, the production processes canbe divided into vacuum hot pressing, hot pressing under atmosphere,vibratory hot pressing, balanced hot pressing, hot isostatic pressing,reaction hot pressing and ultrahigh-pressure sintering. Since heatingand pressing are carried out simultaneously in hot-press sintering, thepowder is in a thermoplastic state, which is favorable for theproceeding of the contact diffusion and flowing mass transfer process ofthe grains, and therefore forming pressure is only one tenth of that incold pressing; furthermore, sintering temperature can be decreased,sintering time can be shortened, and thereby the crystalline grain isinhibited from growing up, obtaining a product with a fine crystallinegrain, high compactness and good mechanical and electrical properties.

In order to adopting the hot-press sintering process to prepare amoderator material, refer to FIG. 10, a hot-press sintering device 200mainly includes a heating furnace 201, a pressing assembly 202 arrangedin the heating furnace 201, a mold 203, powders or powder compacts 204loaded in the mold 203, and a controller 205. The heating furnace 201normally adopts electricity as a heat source, and a heating element ismade of an SiC, MoSi or nichrome wire, a platinum wire, a molybdenumwire, etc. The pressing assembly 202 is required to be steady and slowin speed, constant in pressure keeping and flexible in pressureregulation, and normally, there are a lever type and a hydraulic type.According to the requirement of material properties, pressurizedatmosphere can be air, as well as reducing atmosphere or inertatmosphere. The mold 203 is required to be high in strength, hightemperature resistant, oxidation resistant and not sticking with ahot-pressed material, the thermal expansion coefficient of the mold 203should be consistent with or approximate that of the hot-pressedmaterial, and as a preference, a graphite mold is adopted in the presentembodiment. The controller 205 ensures that the hot-press sinteringdevice 200 is under the condition of normal operation. The so-callednormal operation means that the spark plasma sintering device does notgenerate visual, tactile or auditory warning signals perceivable by thehuman being, such as the shining of a warning indicator light, thesounding of the warning indicator light, warning indicator vibration andthe like.

Taking adopting the hot-press sintering process to prepare a targetmoderator from MgF₂ as an example, the production process flow generallyincludes the following steps: preparation of MgF₂ material—grinding andscreening of material—transferring into mold—high-temperaturesintering—high-temperature hot-press sintering—cooling and discharge—hotisostatic pressure—high-temperature sintering—cooling anddischarge—grinding, polishing and bonding—finished product.

As a preference, the preceding powder treatment step and the succeedingtreatment step for sintering completion are omitted here. The hot-presssintering process includes the following steps: filling the mold 203with an appropriate amount of powders or powder compacts 204; switchingon the hot-press furnace 201 to preset pressure and temperatureparameters; moving the pressing assembly 202 to press the powders orpowder compacts 204 in the mold 203; controlling, by the controller 205,the hot-press sintering device 200 to be under the condition of normaloperation; and switching on power to sinter the powders or powdercompacts 204 into blocks.

It needs to be further explained that the step “moving the pressingassembly 202 to press the powders or powder compacts 204 in the mold203” in the hot-press sintering process can be prepressing or carriedout as the power is switched on, that is, the step “moving the pressingassembly 202 to press the powders or powder compacts 204 in the mold203” and the step “switch ing on power to sinter the powders or powdercompacts 204 into blocks” are integrated.

Some parameters of crystal growing, spark plasma sintering and hot-presssintering are listed in the following table for comparison. As amaterial which more easily to be used for the moderator in the beamshaping assembly for neutron capture therapy disclosed in the presentdisclosure, a moderator material which is prepared by powder sinteringis suggested to be used here, especially under the prerequisite ofneeding to produce a moderator with dimensions among which the maximumsecond diameter D2 is up to 100 cm, and see the specific descriptionbelow.

TABLE 7 Comparison between crystal growing and powder sinteringprocesses Process Process Material Dimensions Time Cost difficultyCrystal MgF₂ Monocrystal 10-20 cm Half a year 5,000,000 yuan Difficultgrowing (maximum diameter) or so or so AlF₃ Monocrystal 10-20 cm Half ayear 5,000,000 yuan Difficult (maximum diameter) or so or so CaF₂Monocrystal 10-20 cm Half a year 5,000,000 yuan Difficult (maximumdiameter) or so or so Spark MgF₂ According to actual 1 month or 500,000yuan Easy plasma dimensional requirement so or so sintering AlF₃According to actual 1 month or 500,000 yuan Easy dimensional requirementso or so CaF₂ According to actual 1 month or 500,000 yuan Easydimensional requirement so or so Vacuum MgF₂ According to actual 2months 1,000,000 yuan Easy hot-press dimensional requirement or so or sosintering AlF₃ According to actual 2 months 1,000,000 yuan Easydimensional requirement or so or so CaF₂ According to actual 2 months1,000,000 yuan Easy dimensional requirement or so or so Hot isostaticMgF₂ According to actual 2-2.5 500,000 yuan Easy pressure dimensionalrequirement months or so sintering AlF₃ According to actual 2-2.5500,000 yuan Easy dimensional requirement months or so CaF₂ According toactual 2-2.5 500,000 yuan Easy dimensional requirement months or soNote: Except the main materials of powder, the above table omits 0.1 to5 percent of ⁶LiF powder which is added in each main material, andalthough the above table only lists the three moderator materials, i.e.MgF₂ + LiF, AlF₃ + LiF and CaF₂ + LiF, and adopts the parameters of theabove processes for comparison, as known well by those skilled in theart, other moderator materials, such as Al₂O₃ + LiF, can also be easilycompared.

Known from the above table, although the density of the moderatormaterial prepared by adopting the crystal growing method can approximatetheoretical density, for example, reaching 99.99 percent of theoreticaldensity, as the size of a monocrystal is small, in order to achieve atarget large-size moderator material, multiple monocrystals need to beput together, other steps, such as mirror polishing, may need to becarried out on the moderator material in the process as well, and as aresult, not only is a great deal of time consumed, but also both thecost and the process difficulty are great.

The density of the moderator material prepared by adopting the powdersintering method can also reach 80 to 100 percent of theoreticaldensity. As a preference, the density of the moderator material reaches99.99 percent of theoretical density. While there is almost nodifference between the theoretical density and the theoretical densityof the moderator material obtained by the crystal growing method, it hasremarkable advantages in terms of obtained dimensions, time, cost andprocess difficulty. The actual dimensions of a moderator materialprepared by adopting spark plasma sintering can be obtained according torequirement, one method can customize a desirable mold, the other methodadopts an ordinary mold, such as a mold which is 70 cm in diameter and 2cm in thickness, and multiple pieces are then put together, and underthe premise that both the cost and the process difficulty are about thesame as that of vacuum hot-press sintering and hot isostatic pressuresintering, its production time is only about one month.

The above illustrates and describes basic principles, main features andadvantages of the present disclosure. Those skilled in the art shouldappreciate that the above embodiments do not limit the presentdisclosure in any form. Technical solutions obtained by equivalentsubstitution or equivalent variations all fall within the scope of thepresent disclosure.

What is claimed is:
 1. A powder sintering device for a moderator,wherein the powder sintering device is configured to turn the materialof the moderator into powder sintered blocks from powders or powdercompacts, and the powder sintering device is a hot-press sinteringdevice.
 2. The powder sintering device according to claim 1, wherein thehot-press sintering device comprises: a mold, wherein a predeterminedamount of the powders or powder compacts are loaded in the mold; aheating furnace for presetting pressure and temperature parameters; apressing assembly arranged in the heating furnace, wherein the pressingassembly is moved to press the powders or powder compacts in the mold;and a controller for controlling the normal operation of the hot-presssintering device.
 3. The powder sintering device according to claim 2,wherein the pressing assembly is a lever type or a hydraulic type. 4.The powder sintering device according to claim 2, wherein pressurizedatmosphere is provided in the pressing assembly, the pressurizedatmosphere is air, reducing atmosphere or inert atmosphere.
 5. Thepowder sintering device according to claim 2, wherein the heatingfurnace adopts electricity as a heat source, and a heating element ismade of an SiC, MoSi or nichrome wire, a platinum wire, a molybdenumwire.
 6. The powder sintering device according to claim 2, wherein themold is a graphite mold.
 7. The powder sintering device according toclaim 1, wherein the powders or powder compacts is prepared by mixing amixture containing one or more of PbF₄, Al₂O₃, AlF₃, CaF₂ and MgF₂ and a⁶Li element-containing material accounting for 0.1 to 5% in percentageby weight of the mixture.
 8. The powder sintering device according toclaim 7, wherein the ⁶Li element-containing material is ⁶LiF or ⁶Li₂CO₃.9. The powder sintering device according to claim 1, wherein the powdersor powder compacts is prepared by mixing MgF₂ and ⁶LiF accounting for0.1 to 5% in percentage by weight of the MgF₂.
 10. The powder sinteringdevice according to claim 1, wherein the multiple powder sintered blocksare connected to form the moderator and a density of the moderator is 80to 100 percent of theoretical density.
 11. A powder sintering device fora moderator, wherein the powder sintering device is configured to turnthe material of the moderator into powder sintered blocks from powdersor powder compacts, and the powder sintering device is a spark plasmasintering device.
 12. The powder sintering device according to claim 11,wherein the spark plasma sintering device comprises: a first electrode;a second electrode; a conductive mold arranged between the firstelectrode and the second electrode, wherein a predetermined amount ofthe powders or powder compacts are loaded in the mold; a pulse currentgenerator for providing pulse current for the mold and generatingplasma, and thereby the surfaces of the powders or the powder compactsare activated and heat; a pressing assembly for pressing; and acontroller for controlling the pulse current generator and the pressingassembly.
 13. The powder sintering device according to claim 12, whereinthe pressing assembly comprises a first pressing member and a secondpressing member, at least one of the first electrode and the secondelectrode or at least one of the first pressing member and the secondpressing member is moved, so that the powders or powder compacts in themold are pressed.
 14. The powder sintering device according to claim 12,wherein the conductive mold is made of lead or graphite.
 15. The powdersintering device according to claim 12, wherein the spark plasmasintering device further comprises: a position measurement system formeasuring the position of the pressing member, wherein the positionmeasurement system is controlled by the controller in order to ensurethe normal operation of the position measurement system; an atmospherecontrol system for controlling atmosphere in the mold, wherein theatmosphere control system is controlled by the controller in order toensure that atmosphere in the mold is under the condition of normaloperation; a water cooling system for cooling the spark plasma sinteringdevice, wherein the water cooling system is controlled by the controllerin order to ensure the normal operation of the water cooling system; anda temperature measurement system for measuring temperature in the sparkplasma sintering device, wherein the temperature measurement system iscontrolled by the controller in order to ensure that temperature in thespark plasma sintering device is under the condition of normaloperation.
 16. The powder sintering device according to claim 13,wherein the spark plasma sintering device further comprises awater-cooling vacuum chamber, the water cooling system control thewater-cooling vacuum chamber to cool the spark plasma sintering device.17. The powder sintering device according to claim 11, wherein thepowders or powder compacts is prepared by mixing a mixture containingone or more of PbF₄, Al₂O₃, AlF₃, CaF₂ and MgF₂ and a ⁶Lielement-containing material accounting for 0.1 to 5% in percentage byweight of the mixture.
 18. The powder sintering device according toclaim 17, wherein the ⁶Li element-containing material is ⁶LiF or⁶Li₂CO₃.
 19. The powder sintering device according to claim 11, whereinthe powders or powder compacts is prepared by mixing MgF₂ and ⁶LiFaccounting for 0.1 to 5% in percentage by weight of the MgF₂.
 20. Thepowder sintering device according to claim 11, wherein the multiplepowder sintered blocks are connected to form the moderator and a densityof the moderator is 80 to 100 percent of theoretical density.